The recent research into catalysts largely includes the development of catalyst supports having a large surface area and the preparation of nano-sized catalyst metals. In particular, development of novel catalyst support materials having a large surface area with the low preparation cost is regarded as having a very high added value.
Porous carbon materials have high industrial availability, and are currently utilized in a variety of fields. For example, these materials are employed as selective adsorbents in separation, adsorption removal and gas storage, electrode materials in batteries, fuel cells, high-capacity capacitors, etc., and catalyst supports or catalysts in main catalyst processes.
In accordance with IUPAC (International Union of Pure and Applied Chemistry), porous carbon materials may be classified into micropores (pore size<2 nm), mesopores (2 nm<pores size<50 nm), and macropores (pore size>50 nm), depending on the pore size. Commercially available porous carbon is exemplified by activated carbon mainly having micropores, and the basic concept of a conventional pore formation process is that pores are introduced using an oxidizable gas or a corrosive compound while carbonizing various organic materials, including coal, petroleum pitch, wood tar, fruit shells, a variety of polymers, etc.
The activated carbon mainly having micropores has a very high specific surface area and pore volume, with significant adsorption capacity. However, the drawbacks of microporous carbon have been pointed out, which include a drastically lowered molecular mass transfer rate due to space constraints of excessively small pores, significant loss of electrical conductivity due to large surface functional groups and structural defects, easy deformation or breakdown of the porous structure due to high-temperature treatment, etc. In particular, a compromise between increasing an active area through a high specific surface area and appropriately maintaining electrical conductivity may be cited as the big issue in electrochemical applications. Further, in regard to efficient access of ions and molecules, problems related to the control of pore sizes have become a major issue. With the goal of solving the drawbacks of the activated carbon mainly having micropores and of achieving the requirements in specific applications including removal of offensive odors and VOC (Volatile Organic Chemical) and selective adsorption of large molecules such as protein, etc., research and development into preparation and application of mesoporous carbon materials is thoroughly ongoing these days.
The preparation of mesoporous carbon includes a template method including placing a polymer or a monomer into a ceramic template having mesopores, performing heat treatment for carbonization, and removing the ceramic template using acidic treatment and alkaline treatment. The porous carbon thus prepared is advantageous because the mesopore size distribution is very uniform, and the total pore size may be easily controlled. However, the template method is problematic because mass production of a template having a precisely controlled microstructure, for example, zeolite or mesoporous silica, is difficult, and the price thereof is high. Furthermore, the removal of the template after the preparation of mesoporous carbon is performed using hydrofluoric acid, which is expensive and complicated. Hence, mass production of mesoporous carbon or use thereof as a universal material has suffered.
Another preparation method includes a movable template method including kneading nano-sized ceramic particles with an organic precursor, performing heat treatment for carbonization at an appropriate temperature, and removing the ceramic nanoparticles using an acid and an alkali. Porous carbon having a uniform pore distribution corresponding to the conventional template method may be prepared using ceramic nanoparticles having a particle size uniformly controlled in the range of from ones of to tens of nanometers, and in consideration of the preparation cost and the mass production, methods are being developed which facilitate use in large amounts and are favorable in terms of price because they use particles such as MgO, CaCO3 and so on, in which the particle size is slightly non-uniform and the preparation and removal treatments are comparatively simple. In this case, there should be devised methods able to prevent non-uniform particle distribution (pore distribution according thereto) due to a difference in density of materials in the course of the preparation process, including using nanoparticles which are a template in an amount equal to or more than the amount of the carbon material, or using a surfactant to stabilize the dispersion of particles, etc.
Recently a so-called soft template method which is used to solve the problems of the conventional metal oxide template method is receiving attention thanks to its self-assembly properties using amphiphilic molecules such as a surfactant or a block copolymer. This method is advantageous because the synthesis process is very simple and uniform mesopores may be introduced, but is problematic because the price burden of the block copolymer, etc., cannot be overcome by the current technique.
Catalyst supports having mesopores are expected to be suitable for use in electrode catalysts and the like for fuel cells in terms of efficient access of reactants and products, in particular, liquid materials, and many cases in which performance of fuel cells has been improved thereby have been reported. In order to achieve physical and chemical adsorption with catalysts, it is known that the specific surface area, pore size, pore distribution, pore shape and surface functional groups of the catalyst supports for fuel cells have an influence on high dispersion of catalysts and catalytic activity, and also electrical conductivity, chemical durability, mechanical strength, etc., are required. Although carbon black and carbon materials having nanostructures of various shapes have been utilized to date as the catalyst supports for fuel cells, lots of improvements thereof are required to develop inexpensive catalysts having high activity.
The present inventors have performed studies on catalyst supports having mesopores to develop catalysts having high activity while requiring low costs, and thus have discovered that a porous carbon material having a large number of pores with a unique mesoporous structure may be prepared by forming a carbon film on the surfaces of ceramic nanoparticles, mixing the ceramic nanoparticles with a carbon precursor, and performing heat treatment for carbonization, thus culminating in the present invention.